It has been known for some time that anhydrous aluminum chloride can be electrolytically reduced at low temperatures of about 700.degree.-750.degree. C. to produce aluminum and chlorine with less energy consumption than with Hall technology reduction of alumina. Over the years a number of processes have been described to produce aluminum chloride suitable for electrolytic production of aluminum. Some processes, such as those disclosed in U.S. Pat. Nos. 4,039,647 and 4,039,648, involve the direct chlorination of aluminum oxide materials derived from the Bayer Process under reducing conditions in a molten state. Other chlorination processes involve the chlorination of Bayer Process alumina using a solid carbon reductant, such as partially calcined coke, as taught and described in U.S. Pat. No. 4,284,607. A principal source of alumina in all cases, however, has been Bayer Process alumina. In the Bayer Process bauxite is mixed with a hot concentrated sodium hydroxide solution to dissolve the alumina and separate it from the other major components ("impurities") of the bauxite (silica, iron oxide and titania). The dissolved alumina is then crystallized as alumina trihydrate, .alpha.-Al.sub.2 O.sub.3.3H.sub. 2 O. Because the alumina trihydrate is formed in a sodium hydroxide environment, it contains a significant amount of soda (usually 0.4 to 0.6%; all percentages herein are by weight unless otherwise stated). This high soda level is also carried over to the product alumina. A high soda level in the product alumina is undesirable because of its effect on the operation of the fluid bed chlorination process to produce anhydrous AlCl.sub.3. Soda consumes valuable chlorine as it is chlorinated, and the products of the chlorination of soda (NaCl and NaAlCl.sub.4) accumulate in the fluid bed, adversely affecting process operation and requiring frequent and costly shut-downs for cleaning of the bed. The high soda content alumina also causes difficulties in the separation step of entrained unreacted bed material (alumina and/or coke) and AlCl.sub.3 from soda chlorination products.
Other chlorination processes have been described for the chlorination of relatively high purity alumina. The alumina is typically produced from the calcination of aluminum chloride hexahydrate, (AlCl.sub.3.6H.sub.2 O; "ACH"). For instance, in U.S. Pat. Nos. 4,465,566 and 4,465,659 assigned to the assignee of present application, ACH derived from acid leaching of aluminous material, such as clay, is partially calcined to produce a product which has low levels of soda and is active toward calcination. Because the ACH is formed in an acid environment, the alumina produced from its calcination has low levels of soda, commonly no more than 0.02%. Even though this alumina source is quite satisfactory for chlorination and production of anhydrous AlCl.sub.3, the acid leaching of the aluminous raw materials is energy and capital intensive compared to caustic leaching of bauxite. It would therefore be advantageous to have a process for the production of anhydrous AlCl.sub.3 that combines the economics of Bayer Process and the low soda levels in alumina of the acid leaching processes.